We propose to continue using and extending our studies on the contractile properties of single human motor units, using the new microneurographic technique we have developed during the past three years. This involves stimulating individual motor axons at a site distant from the muscle. This method has provided the first accurate, stable and repeatable measurement of both twitches, and also tetanic responses to a wide range of stimulus frequencies, delivered before and after a standard fatigue test (Burke, 1973). Results, so far, have been obtained only for thenar motor units; but their contractile properties showed distinct differences compared with those of other mammalian species, when both were tested in the same way. For example, these units could not be classified physiologically into different types using conventional criteria. For example, twitch contractile speed measures were unimodally distributed between units, and not correlated with fatigue indices. Furthermore, no units were found with fatigue indices in the FF range, and most weak (small) units were not both slow and fatigue resistant. Thus, if similar results are obtained from other muscles this will raise questions about the validity of interpreting whole human muscle responses, and their changes with fatigue in terms of oat data; and also, about their recruitment order and rate coding behavior in voluntary contractions. Our results also confirmed previous reports, obtained from cat data (Calancie & Bawa, 1985), of substantial errors in twitch data, as usually recorded in man by spike-triggered averaging. We plan now to: (1) use our new techniques to make similar measurements from motor units in postural and other hand muscles; (2) seek particularly units with fatigue indices in the FF range, which may have been absent in our thenar unit population; and (3) examine changes in unit contractile properties following fatigue from voluntary contraction, if possible. Experiments will also be done on whole human muscle to examine motoneuron firing rate regulation during fatigue. Our data generally favor the view that fatigue initiates an inhibitory reflex mediated by group III/IV muscle afferents, triggered by metabolite accumulation in the muscle. In contrast, Hagbarth's group (1990) suggests that the decline in firing rates is caused mainly by withdrawal of motoneuron facilitation by spindles during fatigue. New experiments are planned, some also involving microneurography, to discriminate between these two hypothesis, and to assess the relative roles of both mechanisms.